(1) Field of the Invention
This invention relates to a shoe press apparatus of a paper machine which is provided, for example, in a dewatering stage of the paper machine for pressing wet paper in a traveling state and a paper production method.
(2) Description of the Related Art
FIG. 7 is a view showing a shoe press apparatus of a conventional paper machine and is a sectional view taken along a plane of the shoe press apparatus perpendicular to the apparatus width direction (perpendicular direction to the plane of the figure). Such a shoe press as just mentioned is provided, for example, in a dewatering stage of the paper machine.
Referring to FIG. 7, the conventional shoe press apparatus shown includes a shoe module 120, and a counter roll 4 provided in an opposing relationship to the shoe module 120 and covered on the surface thereof with a rubber or a metal plate or formed using a metal cell having a high corrosion resisting property. Wet paper (which is sometimes referred to simply as paper) 1 sandwiched by a pair of felt webs 2 and 3 having a water absorbing property is pressed by a nip formed by the counter roll 4 and the shoe module 120 so that water included in the wet paper 1 is removed to the felt webs 2 and 3 to dewater the wet paper 1. It is to be noted that, in FIG. 7, an arrow mark A1 indicates the direction of rotation of the counter roll 4, and another arrow mark A2 indicates the direction of rotation of a blanket 6 hereinafter described while a further arrow mark A3 indicates the traveling direction of the wet paper 1 and the felt webs 2 and 3.
The shoemodule 120 includes, as principal components thereof, a cylindrical blanket 6, a press mechanism 100 provided fixedly at a position opposing the counter roll 4 inside the blanket 6 for pressing the blanket 6 toward the counter roll 4, and a lubricating oil injection nozzle 21 provided fixedly on the upstream side in the direction of rotation of the blanket 6 with respect to the press mechanism 100 inside the blanket 6 for injecting lubricating oil toward the inner circumferential face of the blanket 6.
The blanket 6 is formed from a flexible member and is driven to rotate by the counter roll 4 when the counter roll 4 is driven to rotate. The press mechanism 100 includes, as principal components thereof, a piston block 101 extending in the apparatus width direction, a pair of grooved portions (recessed portions) 102 and 103 formed on an upper face 101a of the piston block 101, a pair of pistons 104 and 105 fitted in the grooved portions 102 and 103, respectively, a pair of pressurized oil supply paths 106 and 107 formed in the piston block 101 for supplying pressurized oil to bottom portions of the grooved portions 102 and 103 from the outside, and a shoe 109 disposed on the pistons 104 and 105 and extending in the apparatus width direction.
Further, while the piston 105 and the shoe 109 are in contact with each other, a piston bar 108 is interposed between the piston 104 and the shoe 109 such that, if the piston 104 moves in an upward or downward direction, then the shoe 109 smoothly moves in an upward or downward direction around a fulcrum provided by the contact point between the shoe 109 and the piston 105.
Furthermore, an upper face 109a (face opposing the counter roll 4) of the shoe 109 is formed in an arcuately concaved state such that it extends along the surface of the counter roll 4. The shoe 109 is pressed against the counter roll 4 with the blanket 6 interposed therebetween to form a nip between the counter roll 4 and the shoe module 120.
Further, in the shoe press apparatus shown in FIG. 7, the piston 105 is disposed at a position spaced by a distance L1 to the upstream side in the paper traveling direction from a vertical line R0 drawn vertically downwardly from the center 4a of rotation of the counter roll 4 (drawn in parallel to the direction of force exerted by the pistons 104 and 105). Meanwhile, the piston 104 is disposed at a position spaced by another distance L2 (here, L2>L1) to the downstream side in the paper traveling direction from the vertical line R0. Such a positional relationship of the pistons 104 and 105 as described above is set suitably.
Since the conventional shoe press apparatus is configured in such a manner as described above, if pressurized oil is supplied into a pressurized oil supply path 107, then the piston 105 is moved in an upward direction to push up the upstream side portion of the shoe 109 in the direction of rotation of the blanket 6. On the other hand, if pressurized oil is supplied to the pressurized oil supply path 106, then the piston 104 is moved in an upward direction to push up the downstream side portion of the shoe 109 in the direction of rotation of the blanket 6. In short, it is possible to adjust the force of the pistons 104 and 105 to push up the downstream side portion and the upstream side portion of the shoe 109 by varying the oil pressure of the pressurized oil to be supplied to the pressurized oil supply paths 106 and 107, respectively.
For example, in the shoe press apparatus (L2>L1) shown in FIG. 7, if the oil pressure P1 of the pressurized oil to be supplied to the pressurized oil supply path 107 and the oil pressure P2 of the pressurized oil to be supplied to the pressurized oil supply path 106 are set equal to each other (for example, to approximately 6.7 MPa), then such a pressure profile is obtained that the nip pressure applied to the wet paper 1 gradually increases from the upstream side end portion E1 to the downstream side end portion E2 of the shoe 109 in the paper traveling direction such that the highest nip pressure (peak nip pressure) P100 (for example, approximately 6.8 MPa) is obtained at the location of the downstream side end portion E2 of the shoe 109 in the paper traveling direction as seen in FIG. 8(a).
Meanwhile, if the oil pressure P2 is set higher than the oil pressure P1 (for example, P1=approximately 5 MPa, P2=approximately 8.3 MPa), then the gradient of the pressure profile shown in FIG. 8(a) becomes steeper, and for example, such a pressure profile as shown in FIG. 8(b) is obtained. In the pressure profile shown in FIG. 8(b), the peak nip pressure at the downstream side end portion E2 of the shoe 109 in the paper traveling direction has a higher value (for example, approximately 8.5 MPa) P200 than the peak nip pressure P100.
On the other hand, if the oil pressure P2 is set lower than the oil pressure P1 (for example, P1=approximately 8.3 MPa, P2=approximately 5 MPa), then as shown in FIG. 8(c), the gradient of the pressure profile becomes less steep than the gradient of the pressure profile shown in FIG. 8(a). Further, the peak nip pressure P300 at the downstream side end portion E2 of the shoe 109 in the paper traveling direction has a value (for example, approximately 5 MPa) lower than the peak nip pressure P100 shown in FIG. 8(a).
It is to be noted that, if the pressure profile is set such that the nip pressure at the downstream side end portion E2 of the shoe 109 in the paper traveling direction is lower than the nip pressure at the upstream side end portion E1 of the shoe 109 in the paper traveling direction, then water absorbed once from the wet paper 1 into the felt webs 2 and 3 returns to the wet paper 1. Therefore, the pressure profile is generally set such that the nip pressure at the downstream side end portion E2 of the shoe 109 in the paper traveling direction is higher than the nip pressure at the upstream side end portion E1 of the shoe 109 in the paper traveling direction as described hereinabove.
A shoe press apparatus of the type described is disclosed, for example, in U.S. Pat. No. 5,167,768 or U.S. Pat. No. 4,917,768.
Incidentally, a shoe press apparatus generally has a nip having a greater width than that of a nip formed, for example, by a pair of rolls and can apply a comparatively low pressure for a long period of time. Therefore, the shoe press apparatus can dewater the wet paper 1 without consolidating the same and is suitable to dewater bulky paper for which a thickness is required.
On the other hand, where a paper machine is used to produce paper having a comparatively small thickness and a high density such as coated base paper, it is necessary to crush (consolidate) the wet paper 1 with a higher peak nip pressure. It is to be noted that, where such coated base paper is to be produced, if the pressure to be applied to the wet paper 1 is insufficient, then this results in comparatively low interlayer strength of the inside of the wet paper 1, and there is the possibility that the inside of the wet paper 1 may be broken when it is dried (this phenomenon is called “blister”).
Accordingly, it seems feasible, for example, to provide, in order to produce coated base paper, a pair of rolls on the downstream side in the paper traveling direction with respect to the shoe press apparatus to consolidate the wet paper 1. In this instance, however, the scale of the apparatus increases. Therefore, it is desired to make it possible for the shoe press apparatus by itself to be used for production not only of bulky paper but also of coated base paper.
However, with the conventional shoe press apparatus, even if the oil pressure P2 is increased further to increase the peak nip pressure at the downstream side end portion E2 of the shoe 109 in the paper traveling direction, although the peak pressure becomes higher than the peak nip pressure P100 illustrated in FIG. 8(a) as in the case of the pressure profile shown in FIG. 8(b), the acting time of the peak nip pressure is very short, and it is difficult to effectively consolidate the wet paper 1. Further, although also it seems feasible to increase both the oil pressure P1 and the oil pressure P2 to generally increase the nip pressure, in this instance, this requires a very high linear pressure and hence is not efficient.